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Embedded system: Has an OS designed to be flashed to firmware with no dependency on external storage devices and survive for a long time without updates/reboots

Embedded system: Has an OS designed to be flashed to firmware with no dependency on external storage devices and survive for a long time without updates/reboots. RTOS: Any operating system that 100% absolutely guarantees IRQ response time or CPU scheduler time. An embedded system's OS may or may not be realtime. An RTOS may be possible on the standard PC platform. Unikernel: User space program linked with required kernel code and designed to run in kernel mode; also possibly designed with the expectation of a hypervisor. The things that come to mind with this are Xbox and Xbox 360 games. So all three of these to me are not mutually exclusive concepts. A unikernel program running in kernel mode probably doesn't need an OS, but there may be need for a hypervisor which the unikernel may use via some virtualized hardware type interface, and that may essentially be an embedded RTOS.

scrubs 5y ago
Great answer. Thanks
djhworld 5y ago
Forgive my ignorance, does this mean that when your user space program makes syscalls, it doesn't require the CPU to go into ring 0/"protected mode" as your program is already running in ring 0?
- derefr 5y ago
  Yes; but furthermore, the "syscall" doesn't end up happening. You're really just writing a kernel driver calling kernel functions, with a libc-looking (or other-language's-runtime-looking) library abstraction (that can be optimized out at unikernel build time.) Usually this library abstraction also has a userspace actually-does-syscalls backend you can build against, giving you a regular userland process, rather than a unikernel, for low-overhead testing. (In this case, if there's anything that absolutely must load into kernelspace, it may compile to a separate DKMS module, which will then communicate through some form of IPC with your userspace process.) But all that complexity is just for testing; in prod, you just get a monolithic kernel with your code inside (that being either an existing kernel with your code being a driver; or your code being "the kernel", with the library being an exokernel framework.)

scrubs 5y ago

Great answer. Thanks

djhworld 5y ago

Forgive my ignorance, does this mean that when your user space program makes syscalls, it doesn't require the CPU to go into ring 0/"protected mode" as your program is already running in ring 0?

derefr 5y ago
Yes; but furthermore, the "syscall" doesn't end up happening. You're really just writing a kernel driver calling kernel functions, with a libc-looking (or other-language's-runtime-looking) library abstraction (that can be optimized out at unikernel build time.) Usually this library abstraction also has a userspace actually-does-syscalls backend you can build against, giving you a regular userland process, rather than a unikernel, for low-overhead testing. (In this case, if there's anything that absolutely must load into kernelspace, it may compile to a separate DKMS module, which will then communicate through some form of IPC with your userspace process.) But all that complexity is just for testing; in prod, you just get a monolithic kernel with your code inside (that being either an existing kernel with your code being a driver; or your code being "the kernel", with the library being an exokernel framework.)